1. Field of the Invention
This invention relates to multiplexed pulse transmission systems and, more particularly, to methods and apparatus for insuring proper framing of multiplexed pulse streams in such systems.
2. Description of the Prior Art
Multiplexed pulse transmission systems are common in which communication channels are concentrated on a single transmission medium by encoding and multiplexing techniques. Two such systems are found in C. G. Davis et. al. U.S. Pat. No. 3,529,089, granted Sept. 15, 1970, and J. A. Greefkes et. al. U.S. Pat. No. 3,707,604, granted Dec. 26, 1972.
In such pulse data transmission systems, it is common to time-division multiplex the pulses of several data sources in order to better utilize the common transmission facility. Upon receipt of the multiplexed data stream at its destination, the several component data streams must, in general, be individually extracted from the composite. This "demultiplexing" operation necessitates some means of determining which pulses in the multiplexed pulse train correspond to each source.
An often-employed approach requires organizing the source data and additional synchronization data into a repeating data "frame". In one typical and straight-forward example, one pulse at a time is taken from each of N binary data sources and inserted into a particular one of N+1 "time slots" assigned to its source. The extra time slot is dedicated to synchronization or "framing" data and has properties distinguishing it from data in any other time slot. If the position of the framing time slot in the received data can be determined, then the time slot of any data source can be found by measuring from the known framing time slot position, thus allowing data demultiplexing.
Special properties of the framing data are utilized to locate the position of the framing time slot by some form of search among the various time slots; this process is referred to herein as "reframing". The prior art binary framing schemes utilized a preselected pattern of bits in the framing time slot chosen to avoid matching source data sequence. Once the framing slot is located, a typical procedure would be to start a modulo (N + 1) counter counting in synchronization with the received time slots, thereby establishing a correspondence between the occurrence of the framing slot and a particular state, s.sub.f, of the counter. The counter can then be used to control demultiplexing, because a known relationship exists between the states of the counter and the identities of the time slots. When the counter again reaches state s.sub.f, it is assumed that (N + 1) states have been traversed, and that the corresponding time slot contains the next framing bit.
By testing the properties of the data in the succession of supposed framing time slots, using the locations thus predicted by a framing counter, it can be determined whether synchronziation is being maintained between the incoming data and the framing counter; this process is referred to as "misframe detection". The combined processes of synthesizing framing data, misframe detection, and reframing are known as "framing".
In addition to these basic requirements of all framing systems, it is also desirable to provide a framing system which permits speedy reframing, has a low susceptability to false misframe detection, is independent of the source data and involves simple and inexpensive hardware.
In some transmission system, it may not be possible to choose a simple predetermined framing data sequence which does not match source data under all conditions. Source data may vary from "random" sequences at one time, to repetitive idling or code patterns at another time, or even all 1's or all 0's data at another time, due to variations in the activity of a source or in the type of information conveyed by a source. For example, a telephone channel may require transmission of delta-modulated voice, or code patterns representing ringing or dialing, or may be idle. If framing data could be confused with any of these types of data, the possiblility of undetected misframes occurs. This may place strong restrictions on the nature of framing patterns.
If the characteristics of source data cannot be predicted at any given time, then whenever a non-framing time slot is being tested for the presence of framing data, there can be at most a probability of 0.5 that the data will fail the test criterion for each bit tested, assuming binary source and framing data. To achieve this optimum regardless of source data properties, each bit expected in the tested slot (as if it were the framing slot) must have a 0.5 probability of being a "1", independent of any other data. Thus, the data value required to pass the framing criterion, and the data value actually appearing, match with probability 0.5, under any conditions.